1. Field of The Invention
This invention generally relates to a spatial light modulation device (hereunder sometimes referred to as a spatial light modulator) and more particularly to the improvement in display quality and productivity of a liquid crystal device for use in a flat panel display, an optical arithmetic device, a video projector or the like.
2. Description of The Related Art
Generally, a spatial light modulator can perform an incoherent-light-to-coherent-light conversion, as well as a coherent-light-to-incoherent-light conversion. Thus it is considered that a spatial light modulator can be applied to a parallel processing of data and to a direct processing of image data. Further, upon condition that the intensity of light is amplified, a spatial light modulator can be applied to a display system such as a video projector.
An example of such a spatial light modulator is disclosed in SID 86 DIGEST, 1986, Society for Information Displays, pp. 379-382. FIGS. 3(A) and 3(B) illustrate the structure of this spatial light modulator. As shown in these figures, a SiO.sub.2 film d, a dielectric mirror C and a light shielding film 4 are stacked on the side (hereunder referred to simply as the write-light side), on which write light is incident, of a light modulator 5 employing a liquid crystal as a modulation material. (Incidentally, the optical film thickness of the film d is .lambda./4 (incidentally, .lambda. denotes the wavelength of write light and in case of this example, .lambda.=540 nanometers (nm)). Further, let n denote the refractive index of the film d. The thickness t of the film d is obtained by the following equation: t=.lambda./(4n) ). Moreover, a photoconductor 3 is stacked on the write-light side of light shielding film 4. Furthermore, a transparent electrode 2 and a glass substrate 1 are stacked on the write-light side of the photoconductor 3.
On the other hand, a transparent electrode 6 and a glass substrate 7 are stacked on the opposite side (hereunder sometimes referred to as the read-light side), on which read light is incident, of the light modulator 5. Further, an appropriate driving power source 8 is connected to the transparent electrodes 2 and 6.
Hereinafter, an operation of such a spatial light modulator will be outlined. When write light representing desired information is incident on the photoconductor 3 of the spatial light modulator as indicated by an arrow F1, the conductivity of the conductor 3 changes according to the intensity of the write light. Thus the distribution of the conductivity of the conductor 3 corresponds to the distribution of the intensity of the write light. Therefore, a voltage supplied from the driving power source 8 is applied to the light modulator 5 correspondingly to the distribution of the conductivity, namely, to the distribution of the intensity of the write light.
On the other hand, read light is incident on the light modulator 5 as indicated by another arrow F2. However, an electric field corresponding to the distribution of the intensity of the write light affects the light modulator 5. Thus the light modulation of the read light is performed according to this distribution of the electric field. Then, the modulated read light is reflected by the dielectric mirror C and is thereafter outputted as indicated by still another arrow F3.
Incidentally, in case where a liquid crystal (for example, a liquid crystal film) having a crystal or having a support is used as the light modulator 5, a part of or all of the glass substrate is omitted. Further, the light shielding film 4 is used to prevent the read light, which passes through the dielectric mirror C, from reaching the photoconductor 3, disturbing a charge pattern and reducing the contrast between portions of the read image. Further, the light shielding film 4 is provided in the spatial light modulator, if necessary. FIGS. 4(A) and 4(B) illustrate the structure of an example of a spatial light modulator in which the light shield film is omitted.
Hereinafter, the structure of each of first and second conventional spatial light modulators, as well as methods for fabricating the first and second conventional spatial modulators, will be described in detail by referring to FIGS. 3(A), 3(B), 4(A) and 4(B). Incidentally, FIG. 3(A) illustrates the entire structure of the first conventional spatial light modulator. Further, FIG. 3(B) is an enlarged fragmentary sectional view of the first conventional spatial modulator. Moreover, FIG. 4(A) illustrates the entire structure of the second conventional spatial light modulator. Furthermore, FIG. 4(B) is an enlarged fragmentary sectional view of the second conventional spatial modulator.
In case of the first conventional spatial light modulator of FIG. 3(A), a-Si:H photoconductive film 3 having a thickness of 20 micrometers (.mu.m), which is doped with boron (B) of 0.3 parts per million (ppm), is formed on the glass substrate 1, on which an ITO (Indium Tin Oxide) film is also formed as the transparent electrode 2, by performing a chemical vapor deposition (CVD) method. Further, the light shielding film 4 made of CdTe, which is 2 .mu.m in thickness, is formed on the film 3 by effecting a sputtering method. Moreover, the dielectric mirror C is formed by stacking six pairs of alternate SiO.sub.2 film (c-1) and TiO.sub.2 film (c-2), each of which is .lambda./4 (incidentally, .lambda.=540 nm) in optical thickness, on the light shielding film 4, as shown in FIG. 3(B). Finally, a reflection film is made up by adding a SiO.sub.2 film d, the optical thickness of which is .lambda./2, onto the top TiO.sub.2 film (c-2). Incidentally, each of SiO.sub.2 and TiO.sub.2 films is formed by effecting what is called an oxygen ion beam assisted vaporization method. Further, the rate (hereunder sometimes referred to as the film forming rate) of forming a SiO.sub.2 film and that of forming a TiO.sub.2 film are 10 angstrom units/second (.ANG./s) and 1 .ANG./s, respectively. Additionally, the temperature of the substrate at the time of measuring these film forming rates is a room temperature.
Then, what is called a vertical orientating processing is performed on the glass substrate 1, on which the transparent electrode 2, the photoconductive film 3, the light shielding film 4, the dielectric mirror C and the SiO.sub.2 film d are thus formed serially, and on another glass substrate 7, on which another transparent electrode (namely, an ITO film) 6 is formed. Further, these composing elements are stuck together through a spacer (not shown). Subsequently, a nematic liquid crystal such as known under a trade name "EN-38" (manufactured by Chisso Corporation) is injected into the spacer. Thus, the conventional spatial light modulator of FIG. 3(A) is completed.
In case of the second conventional spatial light modulator of FIG. 4(A), a-Si:H photoconductive film 3 having a thickness of 20 .mu.m, which is doped with boron (B) of 0.3 ppm, is formed on the glass substrate 1, on which an ITO film is also formed as the transparent electrode 2, by performing a CVD method. Further, the dielectric mirror B is formed by stacking ten pairs of alternate SiO.sub.2 film (b-1) and Si film (b-2), each of which is .lambda./4 (incidentally, .lambda.=540 nm) in optical thickness, on the photoconductive film 3, as shown in FIG. 4(B). Finally, a reflection film is made by adding a SiO.sub.2 film d, the optical thickness of which is .lambda./2 (incidentally, .lambda.=540 nm), onto the top Si film (b-2). Incidentally, each SiO.sub.2 film is formed by effecting the oxygen ion beam assisted vaporization method. Further, the film forming rate of a SiO.sub.2 film is 10 .ANG./s. Additionally, the temperature of the substrate at the time of measuring this film forming rate is a room temperature.
Then, similarly as in case of the first conventional spatial light modulator, the second conventional spatial light modulator of FIG. 4(A) is finished by using the glass substrate 1, on which the transparent electrode 2, the photoconductive film 3, the dielectric mirror C and the SiO.sub.2 film d are thus formed serially.
Meanwhile, there has been an increasing tendency in thickness of the photoconductor 3 for use in a spatial light modulator. This is due to the necessities of driving the light modulator 5 sufficiently in response to weak write light and of preventing an occurrence of loss of light, namely, preventing light from permeating (namely, passing through) the photoconductor 3. For example, the spatial light modulator disclosed in the Japanese Patent Application No. H4-335596 requires a photoconductor 3 having a thickness of 10 to 30 .mu.m. Therefore, the film forming rate of, for instance, a-Si:H film is small (namely, several micrometers). Consequently, a time required for forming such a film reaches several hours or several tens of hours. Namely, the productivity of the conventional spatial light modulator is very low.
Moreover, in case where a conventional spatial light modulator is used in a video projector or the like, the amplification factor of intensity of light is very large in order to form a bright projection image. Further, when the sensitivity (or sensibility) of the photoconductor 3 is high as described above, leakage of read light sometimes has a bad effect upon picture quality. Especially, leaking light having a wavelength close to that of write light causes reduction in contrast ratio. Generally, a light shielding film having a sufficient thickness is introduced to the conventional spatial light modulator as a countermeasure to solve this problem. However, the introduction of such a thick light shielding film into a spatial light modulator has a drawback in that the resolution thereof is deteriorated. In case of a conventional spatial light modulator disclosed in the Japanese Patent Application Laying-open Official Gazette (Kokai Koho) No. H3-217825, a dielectric mirror made by stacking SiO.sub.2 films and Si films (or Ge films), each of which is .lambda./4 in optical thickness, is employed therein to omit such a light shielding film. This conventional spatial light modulator, however, has a defect in that a sufficient effect may not be obtained if read light is not monochromatic radiation.
The present invention is created to eliminate the above described drawbacks of the conventional spatial light modulators.
It is, accordingly, an object of the present invention to provide a spatial light modulator which has a high sensitivity.
Further, it is another object of the present invention to provide a spatial light modulator which has a high resolution.
Furthermore, it is still another object of the present invention to provide a spatial light modulator which can serve to obtain a high contrast ratio in an output picture.
Moreover, it is yet another object of the present invention to provide a spatial light modulator which has high productivity.